Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries and the like have gained greater interest as, for example, power supplies loaded on motor-driven vehicles or power sources loaded on personal computers, mobile devices and other electric products, etc.
A non-aqueous electrolyte secondary battery comprises an electrode body comprising a positive electrode, a negative electrode, and a porous separator placed between the positive electrode and the negative electrode. The separator works to prevent short circuits associated with direct contact between the positive electrode and the negative electrode. With the micropores of the separator being impregnated with an electrolyte, the separator also serves to form an ion-conducting channel (conduction path) between the two electrodes.
Conventionally, for the separator, a film having a porous resin layer formed from polyethylene (PE) or polypropylene (PP), etc., has been used. In such a separator, when the battery temperature is elevated excessively by an internal short circuit, etc., the resin melts to close the micropores, and the ionic conduction between the two electrodes is blocked. Thus, charging and discharging of the battery are forced to stop, preventing further temperature elevation. Such function of the separator is referred to as a shutdown function. With the separator having a shutdown function, overheating of the battery is prevented.
However, since the resin layer is porous, a temperature increase causes thermal contraction. A large degree of thermal contraction may lead to occurrence of a local short circuit due to a film rupture, etc., which may turn into a more extensive short circuit. In order to prevent short circuits due to thermal contraction of the resin layer, it has been suggested to form a porous heat-resistant layer on the resin layer surface. Patent Document 1 discloses such a separator.
For example, a non-aqueous electrolyte secondary battery used as a vehicle-driving power supply, etc., is expected to have great discharge properties at high rates. In order to increase the high-rate discharge properties, it is important to increase the ionic conductivity of the separator. However, a lower porosity in the heat-resistant layer tends to decrease the ionic conductivity of the separator. In order to increase the high-rate discharge properties, it is preferable that the heat-resistant layer has a high porosity. Patent Document 1 discloses a heat-resistant layer having a porosity of 40 to 60%.